1. Field of the Invention
This invention relates to video processing and more particularly to a vertical scaling process and apparatus.
2. Description of the Prior Art
It is a well known problem to convert raster scanned RGB video to a television format. RGB video in this context includes VGA, SVGA and XVGA; these are examples of component video formats for personal computers. RGB stands for the red, green and blue channels that comprise the complete RGB video signal. Television refers to the NTSC, PAL and SECAM television timing standards which are used in television sets. Common television data formats are composite video and S-video (Y/C). Most commonly, RGB video from personal computers uses a non-interlaced (progressive) scan while television uses interlaced scan. In e.g. NTSC television there are 525 horizontal scan lines of analog video data per frame divided into even and odd interlaced fields of 262.5 lines each. Each NTSC television frame thus includes an odd field and an even field which are interlaced to form one frame. The refresh rate for NTSC television is 30 frames per second (30 Hz) while the fields are refreshed at twice the frame rate which for NTSC is 60 Hz.
Television sets use a technique known as overscan to insure that the picture fills the entire video display, (e.g. picture tube) framed within the bezel. Unfortunately, this overscan technique, when applied to the output video signal from a personal computer, may result in a display image having truncated upper and lower portions as well as truncated left and right portions. For a computer display image such truncation is not acceptable since useful information may appear in the truncated portions. Thus, in order to display a high resolution personal computer video image on a television set, the size of the image must be decreased to make the picture viewable within the bezel. For most commercially sold television sets, vertical overscan is about 12%. Thus if the personal computer output display image is vertically scaled to fit within 400 horizontal scan lines and the length of the active video scaled by 0.88, all of the display image would appear on a television without being truncated due to overscan.
Another problem encountered with displaying VGA video on a television set is caused by the lower frame rate of television, (50 Hz for PAL, 60 Hz for NTSC) contrasted with the relatively high refresh rates of VGA, is typically 60-75 Hz.
U.S. Pat. No. 5,510,843 issued to Keene et al., incorporated herein by reference, describes this situation and an attempted solution. Keene et al, FIGS. 6 and 7A illustrate a filtering technique which provides vertical scaling, which means reducing the number of input lines to a fewer number of output lines in the output video signal. ("Lines" refers to video horizontal scan lines.) Keene et al., describes vertical scaling to reduce the 480 VGA lines to 400 lines for a television output signal. Note that in a typical NTSC television set (or monitor) a total of only 400 horizontal scan lines are available for the actual active video, allowing for the vertical blanking interval and overscan.
Since therefore it is often desirable to scale down or shrink video frames in a horizontal or vertical direction, the scaling typically involves selectively reducing the number of pixels or rows in the frame.
Hence, while prior art solutions are available to the scaling problem, there is need for improvement, especially in vertical scaling, because most prior art vertical scaling methods require the support of external video memory, increasing cost and complexity. One prior approach, using linear interpolation does not include the flicker filter function. A decimating linear interpolator requires two line stores while the following flicker filter requires two line stores. Consequently, four line stores are needed. Also, certain prior implementations produce artifacts such as intensity modulation in the vertical direction when the incoming column of pixels has alternate pixels on/off.